Thick film dielectric structures as exemplified by U.S. Pat. No. 5,432,015 are known and exhibit superior characteristics to that of traditional TFEL displays. High performance red, green and blue phosphor materials have been developed for use with thick film dielectric structures to provide increased luminance performance. These phosphor materials include europium activated barium thioaluminate based materials for blue emission, terbium activated zinc sulfide, manganese activated magnesium zinc sulfide or europium activated calcium thioaluminate based materials for green emission, as well as traditional manganese activated zinc sulfide that can be appropriately filtered for red emission.
The thin film phosphor materials used for red, green and blue sub-pixels must be patterned using photolithographic techniques employing solvent solutions for high resolution displays. Traces of these solutions remaining in the display following photolithographic processing together with reaction of moisture or oxygen present in the processing environment may react chemically with certain phosphor materials that are sensitive to oxidation or hydrolysis reactions to cause performance degradation of the completed display. Continued chemical reactions during operation of the display may cause continued performance degradation thereby shortening the life of the display.
To overcome such performance degradation problems, researchers have proposed the use of various materials in conjunction with phosphor materials including zinc sulfide rare earth metal activated phosphors as disclosed for example in U.S. Pat. Nos. 6,048,616 and 6,379,583.
Ihara et al., (Journal of the Electrochemical Society 149 (2002) pp H72-H75) discloses the use of glass to encapsulate nanocrystalline terbium activated zinc sulfide grains. Such encapsulated nanocrystalline grains led to substantially increased photoluminescence and cathodoluminescence as compared to bulk terbium activated zinc sulfide that was attributed to an increase in the transition probability for the decay of the terbium atom from its excited state. The glass coating prevented loss of sulfur and terbium relative to the zinc content of the particles under electron bombardment, whereas uncoated particles with the same diameter showed a significant loss of sulfur and some loss of terbium under the same conditions. The sulfur loss was due to displacement of sulfur from the zinc sulfide by oxygen. However, this reference teaches that the glass coating method is not applicable to the coating of bulk materials such as deposited films and therefore the use of the coated powders for electroluminescent applications was not considered where the factors controlling luminance are different than they are for photoluminescence or cathodoluminescence. Also, a reduction in the grain size of manganese activated zinc sulfide phosphor films in electroluminescence applications did not facilitate an improvement in luminance, but rather decreased the luminance, showing that a reduction in grain size does not necessarily lead to increased luminance.
Mikami et al., (Proceedings of the 6th International Conference on the Science and Technology of Display Phosphors (2000) pp. 61-4) disclose the use of sputtered silicon nitride layers to encapsulate a terbium activated zinc magnesium sulfide thin film phosphor layer in an electroluminescent device to improve the emission spectrum for use as a green phosphor. Luminosity or luminance stability of the device was not addressed.
J. Ohwaki et al., (Review of the Electrical Communications laboratories Vol. 35, 1987) disclose the use of chemical vapour deposition to deposit silicon nitride on an electron beam deposited terbium activated zinc sulfide phosphor film to improve its luminance stability. The silicon nitride layer was to provide a barrier to prevent moisture incursion into the conventional type of zinc sulfide phosphor. Further, chemical vapour deposition processes are difficult to adapt to large area electroluminescent displays for television and other large format display applications and suffer cost and safety disadvantages associated with the handling of volatile precursor chemicals and remediation of effluent gases required for the processes.
U.S. Pat. No. 4,188,565 discloses the use of oxygen-containing insulator silicon nitride layers for use with a manganese activated zinc sulfide phosphor where the oxygen content in the silicon nitride is between 0.1 and 10 mole percent. It is taught in this patent that silicon nitride that does not contain oxygen is unsatisfactory because it does not form a sufficiently strong bond with the phosphor material to prevent delamination. The above noted patent further teaches deposition of the oxygen doped silicon nitride by the use of a sputtering process in a low-pressure atmosphere of nitrogen or a nitrogen-argon mixture containing nitrous oxide. A second insulator layer in combination with the oxygen doped silicon nitride layer is also used to prevent degradation of the phosphor material due to reaction with ambient moisture.
U.S. Pat. No. 4,721,631 discloses deposition of a silicon nitride layer or a silicon oxynitride layer on top of a manganese activated zinc sulfide phosphor film using a plasma chemical vapour deposition method. In this method the process gas for the deposition includes nitrogen and silane rather than ammonia and silane in order to exclude hydrogen from the process since hydrogen can react with sulfur bearing phosphor materials to form hydrogen sulfide, thereby degrading the phosphor performance. It is disclosed that silicon nitride layers deposited using the ammonia free plasma chemical vapour deposition process enable equivalent performance results with manganese activated zinc sulfide phosphors to those obtained with sputtered silicon nitride layers, whereas silicon nitride layers deposited using ammonia yield inferior results.
U.S. Pat. No. 4,880,661 discloses that a manganese-activated zinc sulfide phosphor film cannot successfully be deposited on top of a silicon nitride film using chemical vapour deposition due to its hydrogen concentration. The hydrogen migrates into the phosphor during thermal annealing of the deposited phosphor, causing degradation by loss of sulfur due to reaction with the hydrogen.
U.S. Pat. No. 4,897,319 discloses an electroluminescent device with a double-stack insulator on either side of a manganese-activated zinc sulfide phosphor layer to improve the luminance and energy efficiency of the device. One of the stack members is silicon oxynitride and the other is barium tantalate. The order of the members are reversed on the two sides with the silicon oxynitride layer in contact with the phosphor film on one side and the barium tantalate oxide layer in contact with the phosphor on the opposite side.
U.S. Pat. No. 5,314,759 discloses an electroluminescent display that includes a terbium activated zinc sulfide phosphor layer deposited by Atomic Layer Epitaxy (ALE) and a layer of samarium doped zinc aluminum oxide.
U.S. Pat. No. 5,496,597 discloses a method for making a multilayer alkaline-earth sulfide-metal oxide structure for electroluminescent displays. The phosphor layer has dielectric layers on each side composed of various materials including aluminum oxide.
U.S. Pat. No. 5,598,059 discloses various phosphors including zinc sulfide doped with terbium and having insulating layers of various materials including aluminum oxide.
U.S. Pat. No. 5,602,445 discloses various phosphors with layered construction and having insulating and buffer layers about the phosphor. In one aspect, zinc sulfide is used to sandwich a calcium chloride or strontium chloride rare earth activated phosphor.
U.S. Pat. No. 5,644,190 discloses the use of insulator layers of silicon oxide on both sides of phosphor layers of various materials including manganese activated zinc gallium oxide and zinc cadmium sulfide activated with silver and indium oxide.
WO 00/70917 discloses an electroluminescent laminate that includes a rate earth activated zinc sulfide material having a diffusion barrier layer of zinc sulfide.
While the aforementioned references and patents may teach the use of a conventional large grained rare earth activated zinc sulfide phosphor with certain types of “barrier” or “insulator” materials” for the purpose of preventing reaction of the phosphor with water from the ambient environment or some other “stabilizing” type function, there remains a need to provide an improved zinc sulfide rare earth activated phosphor that has both improved luminance and a long operating life with minimal degradation.